Does directing Star Wars automatically rule JJ out of also doing another Trek film? Assuming production dates don't clash, is there some rule that you're not allowed to work on both franchises???

No, directing Star Wars is not enough of a reason to keep Abrams away from Trek XIII. Practically speaking however, I awlays suspected Abrams and his cohorts wouldn't be involved with Trek XIII anyway. Paramount will want it in 2016, Trek's 50th anniversary. If Abrams delivers Star Wars by 2015, that doesn't leave very much time to get XIII done. And since his Cohorts took their sweet time getting the STID script done, Paramount will want someone who doesn't take three years to do something they promised would be done "at the end of the month." Hell, though Disney wants Abrams to direct Star Wars, they do not want his cohorts write it.